Sei Whale - Anatomy

Anatomy

The whale's body is typically a dark steel grey with irregular light grey to white markings on the ventral surface, or towards the front of the lower body. The whale has a relatively short series of 32–60 pleats or grooves along its ventral surface that extend halfway between the pectoral fins and umbilicus (in other species it usually extends to or past the umbilicus), restricting the expansion of the buccal cavity during feeding compared to other species. The rostrum is pointed and the pectoral fins are relatively short, only 9%–10% of body length, and pointed at the tips. It has a single ridge extending from the tip of the rostrum to the paired blowholes that are a distinctive characteristic of baleen whales.

The whale's skin is often marked by pits or wounds, which after healing become white scars. These are now known to be caused by "cookie-cutter" sharks (Isistius brasiliensis). It has a tall, sickle-shaped dorsal fin that ranges in height from 38–90 centimeters (15–35 in) and averages 53–56 centimeters (21–22 in), about two-thirds of the way back from the tip of the rostrum. Dorsal fin shape, pigmentation pattern, and scarring have been used to a limited extent in photo-identification studies. The tail is thick and the fluke, or lobe, is relatively small in relation to the size of the whale's body.

Adults have 300–380 ashy-black baleen plates on each side of the mouth, up to 80 centimeters (31 in) long. Each plate is made of fingernail-like keratin, which is bordered by a fringe of very fine, short, curly, wool-like white bristles. The sei's very fine baleen bristles, about 0.1 millimetres (0.004 in) are the most reliable characteristic that distinguishes it from other rorquals.

The sei whale looks similar to other large baleen whales. The best way to distinguish between it and Bryde's whale, apart from differences in baleen plates, is by the presence of lateral ridges on the dorsal surface of the Bryde's whale's rostrum. Large individuals can be confused with fin whales, unless the fin whale's asymmetrical head coloration is clearly seen. The fin whale's lower jaw's right side is white, and the left side is grey. When viewed from the side, the rostrum appears slightly arched (accentuated at the tip), while fin and Bryde's whales have relatively flat rostrums.